The three-dimensional printing process is a solid free-form fabrication process, i.e., a process which constructs a three-dimensional article on a layer-by-layer basis from a build material using a computer representation of the article. The build material is a powder, for example without limitation, metal powders and ceramic powders. A layer of the build material is spread across a vertically movable platform. A solution containing a binder is ink-jet printed onto the build material layer in the pattern of the first of a series of cross-sectional slices of the article that is to be built. (A binder is a substance that acts to bind the powder particles together after the solvent portion of the applied binder solution has evaporated.) The platform is then lowered an amount that is equal to a layer-thickness. Another layer of build material is applied over the first layer and binder solution is ink-jet printed onto the second layer in the pattern of the second cross-sectional layer of the series. This sequence of spread-and-print is continued until the article is constructed. More than one article can be made at a time. Eventually, the article is removed from the surrounding bed of unbonded build material and any unbonded build material that is retained in internal passages of the article is removed. The three-dimensional printing process is described in more detail in U.S. Pat. No. 6,036,777 to Sachs, issued Mar. 14, 2000.
Post-processing operations are typically employed to enhance the physical properties of an article formed by the three-dimensional printing process. Typically, powder articles constructed of such build materials are heated to cause the powder particles to sinter together and then the sintered articles are cooled. The sintering effectively unites the individual powder particles together into a single, cohesive unit. As a result, the sintered article obtains substantial additional structural strength. Additional operations, for example without limitation, infiltration or heat treating, may be employed to further modify the structural strength of the sintered article.
Prior to the sintering heat treatment, the structural strength of the built article is primarily provided by the binder that was ink-jet printed onto the build material, although minor contributions to the structural strength may be made by other factors such as, for example without limitation, interparticle friction and the mechanical interlocking of powder particles. A curing step may be used to strengthen the binder. Usually, the binder provides the article with sufficient structural strength to permit the article to be removed from the bed of unbonded building material and handled for further processing.
During the sintering heat treatment, the structural strength of the article changes as the temperature increases and the binder is thermally removed from the article. Generally, the thermal removal results from the volatilization of the binder or of its individual components or of some degradation or decomposition product that is formed from the binder as the heating progresses in the heat treatment environment. As the binder is removed, degrades, or decomposes, its contribution to the structural strength of the article eventually decreases, although in cases wherein the binder melts as it is heated, its contribution to the structural strength may temporarily increase as the surface tension of the liquefied binder forms capillary force bonding between the powder particles. Eventually, however, the structural strength decreases again as the binder removal continues and the capillary force bonds disappear. In all cases, a temperature is reached at which binder removal has progressed to the point whereat its contribution to the structural strength of the article becomes insubstantial. Unless interparticle sintering has begun by this temperature, the structural strength of the article is dependent on the restraint provided by such phenomena as interparticle friction and mechanical interlocking of powder particles. In some cases, decomposition products of the binder, e.g., a carbon residue, may add to the interparticle friction or react with the powder particles to contribute to the structural strength of the article. The article's shape may warp if the article's structural strength at any time during the process becomes insufficient to balance the body forces on the article, for example without limitation, gravity and surface tension.
Unfortunately, for some build material powders or article shapes, it is difficult or impossible to obtain sufficient structural strength to avoid shape distortion while the article is being heated up to the sintering temperature. This is especially true for spherical powders because their smooth, unfeatured surfaces minimize the contributions to an article's structural strength from interparticle friction and interlocking. It is also especially true where the article is thin, has high aspect ratios, or has unsupported overhanging features.
It is therefore an object of the present invention to provide a means for increasing the structural strength of a three-dimensional printed article during the heating portion of the sintering cycle.
As described below, the present invention fulfils this object through a combination of inkjet printable nanoparticle suspensions and three-dimensional printing using a powder build material. Until now, inkjet printable nanoparticle suspensions have been used for printing only on planar substrates or onto previously deposited layers which were initially deposited on planar substrates. For example, Sawyer B. Fuller et al., Ink-Jet Printed Nanoparticle Microelectromechanical Systems, Journal of Microelectromechanical Systems, Vol. 11, No. 1, February 2002, 54-60, teaches inkjet depositing, in a raster-like fashion, multiple layers of 10% by weight metal colloidal nanoparticles suspended in α-terpineol “inks” onto horizontal, flat substrates, which were heated to 100 to 300° C. to flash-evaporate the printed droplets on contact and to make two- and three-dimensional articles up to 400 dried droplet layers thick which were further heat treated to sinter the nanoparticles together into a functional microelectromechanical (“MEMS”) device. The nanoparticles used were gold and silver particles 5 to 7 nanometers in size. The structures built were electrically and mechanically functional MEMS devices. Fuller et al. found that using an unheated substrate caused problems due to uneven wetting. In another example, John B. Szczech et al., Fine-Line Conductor Manufacturing Using Drop-On-Demand PZT Printing Technology, IEEE Transactions On Electronics Packaging Manufacturing, Vol. 25, No. 1. January 2002, 26-33, teaches inkjet printing nanoparticle fluid suspensions of 1-10 nanometer 30% gold-plus-5% copper or 30% silver-plus-5% copper (by weight) particulates in a toluene carrier fluid in patterns onto horizontal, flat substrates and sintering them at 300° C. to fabricate fine-line electrical circuit interconnect conductors. And in another example, Hsien-Hsueh Lee et al., Inkjet Printing Of Nanosized Silver Colloids, Nanotechnology, Vol. 16 (2005) 2436-2441, teaches inkjet printing lines of 5 to 35% (by weight) silver nanoparticles 50 nanometer in size dispersed in a 50/50 weight percent diethylene glycol and water cosolvent carrier vehicle onto horizontal glass slides and then sintering the deposited lines to produce electrical conductors.